The present invention relates to a CMOS image sensor; and, more particularly, to an apparatus and method for automatically controlling an exposure time using a counter that counts green pixels according to predetermined ranges, reducing a burden of hardware and software and improving a performance of the CMOS image sensor.
A conventional image sensor has controlled an illuminance of image using software. Software calculates a current illuminance using a captured image value. Then, if the calculated illuminance is judged to be very bright or very dark, an external controller adjusts the illuminance of the image. Thereafter, illuminance of the adjusted image is again read to judge the illuminance of the image.
In such a conventional method for controlling illuminance through software, however, the actual judgement of the illuminance is completed after processing at least two images from the image sensor. Therefore, a problem may occur and the software is burdened with an increased amount of calculation. In addition, the calculated results should be again programmed through a programming interface and the programmed results is applied to the image capturing, so that a considerable delay time is required. Furthermore, in case the image sensor is used in such as CCTV (Closed Circuit TeleVision), where it is difficult for software to control the illuminance, it may not cope with the change of situation initially set up.
It is, therefore, an object of the present invention to provide an apparatus and method for automatically controlling an exposure time using a counter that counts green pixels according to predetermined ranges, reducing a burden of hardware and software and improving a performance of the CMOS image sensor.
In accordance with an aspect of the present invention, there is provided an apparatus for automatically controlling an exposure time in a CMOS image sensor, comprising: a data-estimation means for estimating the green pixel values according to respective predetermined ranges to generate count control signals; a counting means for performing a counting operation in response to the corresponding count control signals, wherein the counting means includes a first counter for counting green pixels having pixel values within a predetermined reference range, a second plurality of counters for counting green pixels having pixel values greater than the predetermined reference ranges according to the respective predetermined ranges, and a third plurality of counters for counting green pixels having pixel values smaller than the predetermined ranges according to the respective predetermined ranges; a first means for adding and comparing count values from the second plurality of counters to generate a first total count value and a first maximum count value; a second means for adding and comparing count values from the third plurality of counters to generate a second total count value and a second maximum count value; a control means for comparing count values of the first counter, the second plurality of counters and the third plurality of counters to generate a update control signal and an exposure control rate; and an exposure time control signal generating means, in response to an exposure control mode signal and the update control signal, for generating a next exposure time control signal.
In accordance with another aspect of the present invention, there is provided a method for automatically controlling an exposure time in a CMOS image sensor, comprising the steps of: a) estimating green pixel values and counting green pixels according to respective predetermined ranges; b) calculating a first total count value and a first maximum count value of green pixels having pixel values greater than a predetermined reference range and a second total count value and a second maximum count value of the green pixels having pixel values smaller than the reference ranges; c) comparing the first total count value with the second total count value; d) comparing a third total count value of the green pixels having a pixel value within the predetermined reference range with the first maximum count value if the first total count value is greater than the second total count value, and comparing the third total count value with the second maximum count value if the second total count value is greater than the first total count value; e) capturing a next image according to a current exposure time control signal if the third total count value is greater, and calculating a next exposure time control signal and capturing the next image according to the next exposure time control signal if the third total count value is smaller; and f) repeating the steps a) to e) until the final completion of capturing image.